Complementary metal-oxide semiconductor (CMOS) sensors are gaining in popularity over traditional charged-coupled devices (CCDs). A CMOS sensor typically comprises an array of pixels formed in a pixel region and logic circuits in a logic region on the sensor. A pixel region may comprise a light-sensitive CMOS circuit to convert photons into electrons. The light-sensitive CMOS circuit may comprise a photo-sensitive element, such as a photo-diode, formed in a silicon substrate. As the photo-sensitive element is exposed to light, an electrical charge is induced in the photo-sensitive element. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. The electrons are converted into a voltage signal in the pixel region and further transformed into a digital signal and processed by logic circuits in the logic region on the sensor.
In addition to the photo-sensitive element, the pixel region may comprise a plurality of transistors such as a transfer transistor, a reset transistor, a source follower transistor, or a select transistor, connected to the photo-sensitive element. The logic region may comprise a plurality of transistors as well. A transistor comprises a gate made of electrically conductive materials surrounded by insulative materials such as spacers alongside the gate to insulate the gate from other conductive materials. Various features of a transistor such as the gate and the spacer may be made using photolithography techniques, where photoresist materials may be deposited and patterned to produce the intended features.
With the increasing shrinkage of the sizes of the devices such as the gates of the transistors, the thickness of the photoresist materials used in the photolithography process may be reduced as well. Photoresist materials with reduced thickness can cause damages to spacers of transistors in a pixel region during the etching process after the photoresist materials have been patterned. New methods and apparatus are needed to reduce damages to spacers of transistors in a pixel region, which can efficiently reduce the pixel width, while improving dark current and white pixel problems for the pixels.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.